It is known that a layer-by-layer (LbL) coating can be assembled on a substrate from specifies having opposite electric charges. More precisely, positively and negatively charged polyelectrolytes can be alternatively deposited on a substrate.
For this purpose, at least two different polyelectrolyte solutions opposite charges, or a polyelectrolyte solution and a nanoparticles solution having opposite charges, may be used to form the LbL coating.
By selecting the materials of the layers and the deposition process conditions, such films can be anti-reflective, hydrophilic or superhydrophilic, hydrophobic or super hydrophobic.
LbL coatings having hydrophilic properties may also have anti-fog properties. US patent application 2007/0104922 discloses superhydrophilic LbL coatings that can be anti-reflective and anti-fog, such as poly(allylamine hydrochloride)/SiO2 LbL coatings.
However, the LbL coatings exhibit generally poor mechanical properties due to a high porosity, especially poor adhesion to substrate, either naked or already coated by classical hard coat. Most of the LbL coatings are easily wiped off by dry or wet cloth, showing very poor adhesion to the substrate or poor cohesion of the coatings themselves.
It has already been proposed to increase mechanical properties of LbL coatings by calcination treatment, generally at high temperatures (typically around 550° C.). A disadvantage associated to this solution is that it cannot be applied on any organic substrates and it is only adapted to substrates that can withstand high temperature like glass substrates.
US patent application 2008/0038458 discloses a hydrothermal calcination of TiO2/SiO2 coatings, typically at pressure in the range of 10 psi to 30 psi (i.e. 7×104 Pa to 21×104 Pa) at temperature less than 500° C., in order to improve the abrasion resistance of the coatings. Such hydrothermal treatment affects the anti-fog properties of the coating.
More recently, it has also been proposed, in international application WO 2012/013739, to improve the cohesion of the LbL layer by a process of manufacturing an optical device comprising a substrate having one main surface bearing hydroxyl groups coated with an intermediate layer comprising at least one hydroxylated and amino-functionalized siloxane oligomer, and forming on this intermediate layer an anti-fog layer-by-layer coating consisting of at least two bilayers obtained by: (a) forming a first layer on the intermediate layer by applying a first composition comprising a polyelectrolyte obtained from carboxyalkylcelluloses or a polyelectrolyte selected from polyacrylic acid (PAA), poly(methacrylic acid) (PMAA), PAA copolymer, PMAA copolymer or mixtures thereof, (b) forming a second layer on the first layer by applying a second composition comprising a polyelectrolyte obtained from polysaccharide including glucosamine units when the first layer comprises a polyelectrolyte obtained from carboxyalkylcelluloses or metal oxide or silicon oxide nanoparticles which are surface functionalized with amino groups when the first layer comprises the polyelectrolyte obtained from PAA, PMAA, PAA copolymer or PMAA copolymer, (c) optionally repeating at least once steps (a) and (b); and (d) crosslinking the layers of the LbL coating by treatment with a composition of a coupling agent. According to this international application, the coupling agent is chosen among carbodiimides of formula N(CH3)2—(CH2)n-N═C═N—CH2CH3, wherein n is an integer from 2 to 6, such as for example 1-ethyl-3 (3-dimethylaminopropyl) carbodiimide (EDC). These coupling agents are used in combination with N-hydrosuccinimide (NHS), sulfohydroxysuccinimides or N-hydroxybenzotriazole to increase coupling efficiency and decrease side reactions. This process involves many steps including a step of applying an intermediate layer on the substrate to promote the adhesion of the LbL coating to the substrate.
This invention provides an alternative and simple approach to enhance the adhesion of the LbL coating to the substrate and/or the cohesion between LbL coating, while keeping the functional properties of coatings such as anti-fog properties.